Various compressed air foam systems (CAFS) have been used in firefighting for quite some time. The original CAFS developments were made by the U.S. Navy during the 1940's as a method to fight fires on ships. In its most basic form, a CAFS is simply a means for mixing air and water with a surfactant in order to produce a water-based foam that is used to extinguish fires. CAFS provides quicker “knockdown” against potent fires, deeper penetration of fuels, and gives firefighters the advantage of making their initial attack against a fire from a significantly greater distance than with a traditional water stream or fog pattern. The bubble structure allows for greater expansion of delivered water surface area, allowing for greater heat reduction compared to equal amounts of plain water. Foam blankets allow for pre-treatment of fuels that are not already involved in the fire, and have less adverse impact on property, as well as helping to prevent damage to evidence used for fire investigations. In fact, some studies have indicated that CAFS increases the effectiveness of water as an extinguishing agent by approximately a factor of five. CAFS may be particularly valuable for rural fire departments, because the use of foam reduces the amount of water required to extinguish a fire, and rural departments are often quite limited in the amount of water that they have available at any particular fire.
A challenge with CAFS design is the maintenance of a correct balance between incoming water and air pressure. If these pressures are not correctly balanced, a low-quality foam, or even no foam at all, may result. The art includes a number of devices intended to manually or automatically balance the incoming water and air pressure in a mixing chamber in order to produce high-quality foam in a CAFS. The inventor hereof has developed previous systems intended to balance air and water pressure to produce high-quality OAFS foam. Examples are described in U.S. Pat. Nos. 5,837,168; 6,086,052; 6,138,994; and 6,217,009, each of which are incorporated herein by reference. Devices based on the descriptions provided in these patents have been marketed by Rowe Industries of Hope, Ark. under the Equalizer® trademark.
One problem raised by the widespread adoption of OAFS is the cost of retrofitting existing water-based systems to employ OAFS equipment. Since rural fire departments are generally not as well funded as many urban fire departments, the replacement of an existing, fully functional water-based pumper truck with one featuring an integrated OAFS unit is often cost prohibitive. For this reason, there is currently a significant demand for retrofitting of standard water-only pumper trucks with OAFS equipment. A problem arises, however, in that the OAFS equipment may not fit neatly within the available space on an existing pumper truck, which complicates the retrofit process and increases the associated cost.
Another problem raised by the adoption of OAFS systems is that there may be certain applications where, even though a OAFS system is available, it is desirable to use only water in fighting a particular fire. For example, this may be desired if solution is not immediately available. It would be advantageous if the OAFS system could simply be deactivated in the field without the removal of any equipment, and most importantly that the resulting unit would be capable of full, unrestricted water flow to the hose nozzle despite the presence of the in-line OAFS system. Likewise, it would be advantageous if the OAFS system allowed for the relatively unrestricted flow of solution rather than water, where “solution” is a combination of water and surfactant without the injection of air to produce foam. Solution has been shown to be more effective than simple water in suppressing certain types of fires, and there are firefighting applications where it is deemed more desirable than OAFS. A further advantage would be to allow for the combination of either water or solution flow with OAFS production to produce a “wet OAFS” flow, thereby giving the firefighter a full selection of operating modes that may be utilized in order to best address a particular firefighting scenario arising in the field. It would also be desirable to develop such a system without the need for complex bypass piping, in order to keep the cost of the retrofit as low as possible and minimize the on-board space required for the retrofit.
What is desired then is an improved OAFS foam generator that can be easily and inexpensively retrofitted to existing liquid-only systems, and is operable in various modes as required for various firefighting applications, without significant lowering of liquid flow in non-OAFS modes despite the presence of the additional OAFS components.